1. Technical Field
The present disclosure relates to systems and methods for ionization current sensing in multiple cylinder internal combustion engines having two or more spark plugs per cylinder.
2. Background Art
Manufacturers continue to improve control of internal combustion engines to enhance fuel economy and performance while reducing feedgas emissions using more sophisticated sensing and processing hardware and software. To improve control of the combustion process, ionization current sensing (or ion sense) uses a bias voltage applied across a sensor positioned within the combustion chamber to generate a current signal indicative of the combustion quality and timing. For spark-ignition engines, one or more spark plugs may be used as an ion sensor with the bias voltage applied across the air gap of the spark plug, or between a spark plug electrode and the cylinder wall.
Spark-ignited internal combustion engines may be configured with ignition systems that feature two or more spark plugs for each cylinder to accommodate flexible fuel applications or to provide more ignition energy for leaner air/fuel ratios to improve combustion and enhance fuel economy, for example. Multiple spark plugs may be powered from a common ignition coil to improve cost effectiveness of these applications. However, multi-plug applications powered by a common ignition coil present various challenges for implementing ion sensing technology. For example, combining or summing ionization current signals from two or more spark plugs or other ion sensors on a common signal line may result in attenuation or cancellation of high frequency components and associated variation in the ion sensing signal that is difficult to correlate with actual combustion performance. Differences in spark durations between two or more spark plugs can mask ion signals for a portion of the engine cycle so that combustion information is unavailable. In addition, electrical and magnetic coupling of the spark discharge can also distort the ion sense signal.